Ceramic with improved high temperature electrical properties for use as a spark plug insulator

ABSTRACT

An insulator including alumina in an amount between about 90 and about 99% by weight and an oxide mixture or glass mixture including Boron Oxide, Phosphorus Oxide, or both Boron and Phosphorus Oxide.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/984,135, filed on Nov. 9, 2004, now U.S. Pat. No. 7,169,723which claims priority to U.S. Provisional Patent Application Ser. No.60/519,395, filed on Nov. 12, 2003, each of which are herebyincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention generally relates to ceramic materials. Moreparticularly, it relates to ceramic materials used in insulators ofspark plugs.

2. Related Art

Spark plugs, glow plugs, and other such devices used in internalcombustion engines are subjected to high temperature environments in theregion of about 1,000° C. In general, a spark plug is a device thatextends into a combustion chamber of an internal combustion engine andproduces a spark to ignite a combustible mixture of air and fueltherein. Specifically, a spark plug typically includes a cylindricalmetal shell having external threads that screw into a portion of theengine and further having a hook-shaped ground electrode attachedthereto at a firing end of the spark plug. A cylindrical insulator isdisposed partially within the metal shell, and extends axially beyondthe metal shell toward the firing end and also toward a terminal end. Aconductive terminal is disposed within a cylindrical insulator at theterminal end of the spark plug opposite the firing end. At the firingend, a cylindrical center electrode is disposed within the insulator andprojects axially out of the insulator toward the ground electrode,whereby a spark plug gap is defined between the electrodes.

In operation, ignition voltage pulses of up to about 40,000 volts areapplied through the spark plug to the center electrode, thereby causinga spark to jump the gap between the center and ground electrodes. Thespark ignites an air and fuel mixture within the combustion chamber tocreate high temperature combustion to power the engine. Unfortunately,the high voltage and high temperature environment within the combustionchamber can degrade components of the spark plug. As the spark plugbecomes degraded, the intensity of the ignition pulse may becomealtered, thereby degrading the quality of the spark. Degradation of thespark plug may be caused by dielectric puncture through the insulatorwhich establishes an alternative electric path and consequently thespark may not reliably jump the gap between the center and groundelectrodes. The quality of the spark effects the ignition of the mixtureof the air and fuel (i.e., the combustion efficiency, combustiontemperature, combustion products) thus, the power output, fuelefficiency performance of the engine, and the emissions produced by thecombustion of the air and fuel may be adversely affected. Due to anincreasing emphasis on regulation of emissions from motor vehicles, theincreasing fuel prices, and modern performance demands, it is desirableto maintain a high quality spark for consistent engine performance andemission quality. The longevity of the spark plug, including, quality ofthe spark, is determined by several factors including the composition ofthe ceramic insulator material.

The ceramic insulator materials used for the insulator are dielectricmaterials. Dielectric strength of a material is generally defined as themaximum electric field which can be applied to the material withoutcausing breakdown or electrical puncture thereof. The dielectricstrength of spark plugs is generally measured in kilovolts per mil(kV/mil). For a given spark plug design, the insulator dimensions arefixed, thus, dielectric strength is frequently expressed as a breakdownvoltage in kV, rather than in kV/mil. A typical value for spark plugdielectric strength for a standard spark plug design used in manyapplications is on the order of about 40 kV at room temperature.Dielectric strength of the insulators used in spark plugs is also afunction of temperature. High temperatures cause an increase in themobility of certain ions allowing the current to more easily leakthrough the ceramic. Any leakage of current leads to localized heatingwhich gradually degrades the resistance of the material to dielectricpuncture. It has been observed that resistance of insulators todielectric breakdown tends to decrease over the life of a spark plug dueto thermal stress on the spark plug cycling under an applied electricfield and due to attendant thermal-electrical fatigue thereof. The exactnature of the microstructural and/or compositional changes are notcompletely understood, but are believed to be associated with localizedheating to temperatures sufficient to bring about partial melting of theceramic material.

Shunt resistance is another measurable property of ceramics,particularly for those used in spark plugs, and is a measure of theelectrical resistance of the material which is generally measured inmegaohms. A typical value for spark plug shunt resistance is on theorder of about 75 to 125 megaohms at an operating temperature of about1000 degrees Fahrenheit. Shunt resistance is typically measured on aspark plug as an electrical resistance of the ceramic insulator betweenthe center electrode and metal shell of the spark plug. Therefore, shuntresistance is indicative of the amount of current leakage through theceramic insulator between the center electrode and metal shell orhousing. Whereas dielectric breakdown tends to be a sudden event, shuntresistance tends to be a continuous, parasitic loss of electrical power.Of course, the lower the shunt resistance, the higher the likelihood ofcatastrophic dielectric failure after the spark plug.

A breakdown in dielectric strength and/or shunt resistance ultimatelyleads to a spark plug with an electrically parallel path between thecenter electrode and metal casing in addition to the path across thespark gap between the center electrode and the ground electrode.Shunting of the spark plug is a condition in which an undesirableparallel conductive path is established between the center electrode andthe metal casing in addition to the path across the spark gap betweenthe center electrode and the ground electrode. However, in the case ofshunting caused by diminished or insufficient shunt resistance, theaffect is may simply degrade the spark performance. This additional patheven if very small has an adverse effect on the quality of the sparkgenerated by the spark plug. Whereas the parallel electric path isgenerally due to dielectric breakdown, the effect is generallycatastrophic and in many cases significantly reducing or completelyeliminating the spark between the center electrode and the groundelectrode. A diminished or insufficient shunt resistance degrades theperformance of the spark plug and consequently the performance of theengine especially over the service lifetime of the spark plug. As statedabove, many times, a degraded shunt resistance will eventually cause acatastrophic failure due to dielectric loss.

As manufacturers continually have increased the complexity and reducedthe size of internal combustion engines, spark plugs are needed thathave a smaller diameter. Also as manufacturers have continuallyincreased the compression ratio of the engine, requiring higher voltagesfor the spark to jump the spark gap. Currently, the size the spark plugis limited from further reduction due to the required dielectricstrength of the insulator over the service lifetime of the plug, whichis directly related to the thickness required for the walls of theinsulator. Another factor limiting size reduction is that moremanufacturers are demanding a longer service lifetime from spark plugssuch as requesting 100,000 mile, 150,000 mile, and 175,000 mile servicelifetimes from spark plugs. The longer the desired service lifetime, thehigher the required dielectric strength. Also, the higher the requiredvoltage, the higher required dielectric strength. Previously to increasethe service lifetime or dielectric strength of a spark plug the walls ofthe insulator were increased in thickness. However, the current demandfor more compact spark plugs for modern engines prevents or limits theuse of thicker walled insulators. Therefore, as engines shrink in sizeand as longer service lifetimes and higher voltages are needed in sparkplugs, a spark plug having an insulator with an increased dielectricstrength and a reduced wall thickness in size is needed.

Therefore, it would be desirable to produce a spark plug using animproved ceramic insulator material with high shunt resistance that isless susceptible to a breakdown in dielectric strength for extendedperiods of time at high voltages and high temperatures and, thus, lesssusceptible to shunting conditions in the spark plug, in order topromote generation of a quality spark and enhanced engine performance.

SUMMARY OF THE INVENTION

The above-noted shortcomings of prior art ceramics are overcome by thepresent invention which provides a ceramic, particularly for use as aninsulator in an ignition device such as a spark plug. Such an insulatorhas improved shunt resistance and dielectric breakdown properties, so asto reduce shunting of the spark plug and thereby improve the quality ofthe spark generated by the spark plug and improved engine performance.

In the present invention, the dielectric strength and the shuntresistance of a ceramic material, such as a spark plug insulator, isimproved through the addition of either Phosphorus Oxide (P₂O₅) or BoronOxide (B₂O₃), or a combination of P₂O₅ and B₂O₃. The additions of P₂O₅,B₂O₃ or a combination of P₂O₅ and B₂O₃ to the ceramic are added to theglass phase of the ceramic. The P₂O₅ is generally added up to 20% byweight of the glass and the B₂O₃ is added up to 15% by weight of theglass. More specifically, P₂O₃ is generally added up to 15% by weightand B₂O₃ is added up to about 12% by weight. When combined, P₂O₅ andB₂O₃ are added up to about 27% by weight of the glass, and morespecifically about 18% by weight of the glass. The dielectric strengthhas been found to increase by up to 5% and the shunt resistance by up to200% through the addition of P₂O₅, B₂O₃ or both P₂O₅ and B₂O₃.

According to one aspect of the present invention, the ceramic includesalumina in an amount between about 90 and about 99% by weight, azirconium containing compound in an amount between about 0.01% and about1% by weight, and an oxide mixture in an amount which ranges betweenabout 1 and about 10% by weight. The zirconium containing compoundpreferably comprises zirconium oxide (ZrO₂). The oxide mixture includesa glass former and a network modifier, wherein the molar ratio of theglass former to the network modifier is in a range between about 0.8:1and 1.2:1. The glass former may comprise SiO₂. The network modifiers maycomprise at least one of MgO, CaO, SrO, BaO, Na₂O, K₂O and Li₂O.

According to another aspect of the present invention, there is provideda spark plug that includes a center electrode, a metal shell, and aninsulator disposed between the center electrode and the metal shell. Theinsulator includes between about 90 and 99% alumina by weight, betweenabout 0.01 and 1% zirconium containing compound by weight, and betweenabout 1 and 10% oxide mixture by weight. The oxide mixture includes aglass former, and a network modifier, wherein the molar ratio of theglass former to the network modifier range between about 0.8:1 and1.2:1. The glass former may comprise SiO₂. The network modifiers maycomprise at least one of MgO, CaO, SrO, BaO Na₂O, K₂O and Li₂O.

In accordance with yet another aspect of the present invention, there isprovided a spark plug including a metal shell, a center electrode, andan insulator disposed in the metal shell and having a central bore withthe center electrode being disposed in the central bore. The insulatorincludes alumina and has a shunt resistance of greater than 1000megaohms at 1000 degrees Fahrenheit.

In accordance with another aspect of the present invention, there isprovided a ceramic material, such as a spark plug insulator havingapproximately 90-99% alumina by weight and a glass including PhosphorusOxide of about 0.05% or greater by weight of the ceramic material.

In accordance with another aspect of the invention, the presentinvention includes an insulator for a spark plug having approximately90-99% alumina by weight and a glass including Boron Oxide of about 1.5%or greater by weight of the glass.

In accordance with another aspect of the present invention, there isprovided an insulator for a spark plug having approximately 90-99%alumina by weight and a glass including Phosphorus Oxide of about 0.05%or greater and Boron Oxide of about 1.5% or greater by weight of theglass.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention willbecome more readily appreciated when considered in connection with thefollowing detailed description and appended drawings, wherein likedesignations denote like elements, and wherein:

FIG. 1 shows a partial fragmentary view of a spark plug having a ceramicinsulator in accordance with the present invention;

FIG. 2 is a main effects plot of mean dielectric peak puncture valuesfor several material composition variables of the ceramic of the presentinvention;

FIG. 3 is a main effects plot of mean shunt resistance values forseveral material composition variables of the ceramic of the presentinvention;

FIG. 4 is a contour plot showing lines of fixed shunt resistance for twomaterial composition variables of the matrix mixture within the ceramicof the present invention;

FIG. 5 is a schematic illustration of a CaO—SiO₂—MgO phase equilibriumdiagram;

FIG. 6 is a partial illustration of a phase equilibrium diagram forCaO—SiO₂—MgO, showing an overlaid contour plot of shunt resistance;

FIG. 7 is a mixture contour plot of shunt resistance of insulator at1000° Fahrenheit;

FIG. 8 is a mixture contour plot of dielectric strength;

FIG. 9 is an overload mixture contour plot of the shunt resistance anddielectric strength;

FIG. 10 is a mixture contour plot of shunt resistance at 1000°Fahrenheit;

FIG. 11 is a mixture contour plot of dielectric strength; and

FIG. 12 is a sectional view of a spark plug insulator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention relates generally to ignition devices for hightemperature applications, such as spark plugs, igniters and other sparkgeneration devices. With reference to FIG. 1, there is shown an ignitiondevice comprising a spark plug assembly 10 for use in an internalcombustion engine (not shown) that generally includes a metal shell 12,a ceramic insulator 14, a center wire assembly 16, and a groundelectrode 18. As commonly known in the art, the shell 12 is a generallycylindrical, electrically conductive component having a hollow bore 20extending along its axial length. Within that bore 20 are a series ofcircumferential shoulders sized to support diametrically reducedsections of the insulator 14. Like the shell 12, the insulator 14 isalso a generally cylindrical component with an elongated axial bore 22.The lower axial end of the insulator 14 comprises a nose portion 24which generally extends out of and beyond the lowermost portion of theshell 12. The insulator axial bore 22 is designed to receive theelectrically conductive center wire assembly 16, which extends theentire axial length of the spark plug 10 and generally includes aterminal electrode 30 at one end and a center electrode 32 at anotherend. Of course, the center wire assembly 16 shown here is simply of atypical embodiment, and could include additional components or havecomponents omitted. The ground electrode 18 is both mechanically andelectrically connected to the lower axial end of the shell 12 and isgenerally formed in an L-shape configuration. The exposed end of thecenter electrode 32 and a side surface of the ground electrode 18 opposeeach other and thereby define a spark gap 34 at a firing end 36 of thespark plug 10.

In operation, the terminal electrode 30 receives a high voltage ignitionpulse from an ignition system (not shown) which travels along the centerwire assembly 16 until it reaches the lower exposed end of the centerelectrode 32. If the pulse has sufficient energy to bridge the spark gap34, a spark is formed between the center electrode 32 and the groundelectrode 18, which in turn is grounded to the engine via the shell 12.The spark ignites a fuel/air mixture which has previously been injectedinto a combustion chamber within the engine, which in turn initiates thecombustion process used to power the engine. The previous explanationwas provided as a general overview of the construction and operation ofthe ignition device. Additional detail about ceramic insulator 14 isprovided in accordance with the present invention.

Insulator 14 of the present invention is an alumina-based ceramic. Ingeneral, alumina-based ceramics comprise fine crystalline Al₂O₃particles in an oxide mixture matrix. The oxide mixture is preferably agenerally amorphous glass matrix, such as various types of silicateglasses, but may also include crystalline materials as part of the oxidemixture. Alumina-based ceramics tend to have relatively high mechanicaland dielectric strength, as well as high electrical resistivity and lowdielectric loss, and are known to retain these properties over arelatively wide temperature range. But, the properties of aluminaceramics are degraded by impurities in the material, thermal fatigue,high voltage, high operating temperatures, and the like. U.S. Pat. No.4,879,260 of Manning indicates that the addition of zirconia to analumina-based ceramic tends to positively affect the mechanical strengththereof, particularly when the zirconia comprises between 0.5 to 1.0percent of the composition by weight.

A focus of the present invention, however, is not to improve mechanicalstrength of an alumina-based ceramics, but rather to provide a ceramicinsulator with improved dielectric strength and shunt resistance, suchthat it is particularly adapted for use in ignition devices. To thisend, experiments were conducted that involved varying the amounts ofalumina, the materials and related amounts that comprise the oxidemixture matrix, and the amounts of zirconia to obtain alumina-basedceramics having a combination of improved dielectric strength or shuntresistance, or both. The amount of alumina was discovered to bepreferably between 90 and 99% of the ceramic composition by weight. Theoxide mixture matrix is composed of a glass former, which is preferablySiO₂ but may also include B₂O₃, P₂O₅, and the like. The oxide mixturematrix is also composed of one or more network modifiers, preferablyCaO, MgO, BaO, and SrO, but may also include other alkaline earth metaloxides, or alkali metal oxides such as Na₂O, K₂O, Li₂O and the like. Thenetwork modifiers may also be known as fluxes. The oxide matrix may alsobe composed of network intermediates, such as Al₂O₃, but may alsoinclude other network intermediates such as TiO₂, ZnO, ZrO₂ and thelike. Since Al₂O₃ is somewhat soluble in the oxide mixture, anequilibrium will exist between the primary Al₂O₃ constituent in the formof Al₂O₃ crystals and Al₂O₃ which is dissolved in the oxide mixturewhere it acts as a network intermediate. The amount of Al₂O₃ that isdissolved in the oxide mixture is very difficult to measureanalytically, but based on the phase equilibrium diagram is believed toconstitute was much as 40% of the oxide mixture by weight forcompositions in the range of the present invention. It was discoveredthat adding certain relatively small levels of a zirconium-basedcompound, such as zirconia (ZrO₂), tends to reduce crystallizationwithin the oxide matrix, as well as improve mechanical strength of theceramic. Crystallization tends to result in higher electricalconductivity. Therefore, the addition of the zirconia tends to lower theelectrical conductivity of the oxide mixture matrix portion of theceramic.

Experiments were conducted to determine the effect of ceramic materialcomposition on the performance of spark plug insulators. The ceramicswere prepared by mixing Alcan C-761 alumina with appropriate amounts ofcommercially available precursor oxide mixture matrix materials, such asEPK kaolin, HuberCarb calcium carbonate, magnesite, dolomite,wollastonite and Yellowstone Talc, and with appropriate amounts of ZiroxZirconia which form oxides upon heating. The powder mixture constituentsused to produce ceramic insulator materials of the invention were ballmilled in an aqueous slurry comprising about 73 percent solids by weightor about 40 percent solids by volume. Batches totaling 5000 grams ofpowder were prepared by ball milling of the materials, followed by spraydrying in a tower spray-dryer. The spray granulate was then compacted bydry-bag isostatic pressing at 8500 psi and formed into the shape ofinsulator 14, and fired at temperatures between 1590 and 1630 degreesCelsius for approximately 3 hours in order to sinter the insulators suchthat the alumina particles are interconnected by the oxide mixturematrix.

The experiments were designed to evaluated three different levels offour variables of material composition. Table 1 below depicts a summaryof the variables used in the experiment. The various materialcompositions specified herein are for purposes of illustrating anddisclosing the present invention, and are not to be construed aslimiting the scope thereof. The experiments were conducted using ninedifferent batches of material that are identified as batches 03-B-17through 03-B-25.

TABLE 1 Variable (Material) Description Levels Values of Levels AluminaWeight Percent of Alumina 3 94%, 95%, 96% Zirconia Weight Percent ofZirconia 3 0.0%, 0.15%, 0.30% CaO Mole Fraction CaO/(RO) 3 0.8, 0.9, 1.0SiO2 Mole Fraction SiO₂/(RO) 3 0.8, 1.0, 1.2

The network modifiers can be identified in general by the designationRO, wherein RO represents the total amount of network modifier presentin the ceramic composition. In Table 1, RO=MgO+CaO. In general, RO isthe sum of all network modifiers present. If the network modifiersinclude CaO, MgO, BaO and SrO, then RO=CaO+MgO+BaO+SrO.

The material compositions of the various batches are reported in acombination of weight percents and molar amounts. The composition of theoxide matrix is reported herein in molar amounts because of the degreeof variation in the atomic weights of the network modifiers that may beused in the present invention. The ratio of atoms in the oxide mixturematrix greatly influences the electrical properties thereof. Since theatomic weights of calcium, magnesium, barium, and strontium varysignificantly, they cannot be readily substituted on a weight basis toachieve the specific compositions of network modifiers discussed herein.Thus, it is preferred to express the components of the oxide mixturematrix in terms of moles rather than in terms of weight.

Accordingly, Table 2A is an experiment matrix that reports the variousexperiment and composition levels used for each material in weightpercent for alumina and zirconia, and in molar ratio for the preferrednetwork modifiers and glass formers. Table 2B, however, reports all ofthe materials in weight percent. Similarly, Table 2C reports in weightpercent, the precursor materials, by batch composition.

TABLE 2A Run DOE Al₂O₃ ZrO₂ MgO CaO SiO₂ Batch ID Order Order wt % wt %mol mol mol 03-B-24 8 1 94.00 0.00 0.20 0.80 0.80 03-B-19 3 2 95.00 0.150.10 0.90 0.80 03-B-17 1 3 96.00 0.30 0.00 1.00 0.80 03-B-20 4 4 94.000.30 0.10 0.90 1.00 03-B-23 7 5 95.00 0.00 0.00 1.00 1.00 03-B-22 6 696.00 0.15 0.20 0.80 1.00 03-B-25 9 7 94.00 0.15 0.00 1.00 1.20 03-B-215 8 95.00 0.30 0.20 0.80 1.20 03-B-18 2 9 96.00 0.00 0.10 0.90 1.20Minimum 94.00 0.00 0.00 0.80 0.80 Maximum 96.00 0.30 0.20 1.00 1.20

TABLE 2B Run DOE Al₂O₃ ZrO₂ MgO CaO SiO₂ Batch ID Order Order wt % wt %wt % wt % wt % 03-B-24 8 1 94.00 0.00 0.48 2.67 2.86 03-B-19 3 2 95.000.15 0.19 2.39 2.27 03-B-17 1 3 96.00 0.30 0.00 1.99 1.71 03-B-20 4 494.00 0.30 0.20 2.51 2.99 03-B-23 7 5 95.00 0.00 0.00 2.41 2.59 03-B-226 6 96.00 0.15 0.27 1.53 2.05 03-B-25 9 7 94.00 0.15 0.00 2.56 3.2903-B-21 5 8 95.00 0.30 0.30 1.69 2.71 03-B-18 2 9 96.00 0.00 0.13 1.592.28 Minimum 94.00 0.00 0.00 1.53 1.71 Maximum 96.00 0.30 0.48 2.67 3.29

TABLE 2C Huber-Carb Alcan Batch Run DOE Yellow-stone Calcium EPK C-761ZIROX ID Order Order Talc Carbonate Kaolin Alumina Zirconia 03-B-24 8 11.59 4.91 4.50 89.00 0.00 03-B-19 3 2 0.63 4.41 4.28 90.53 0.15 03-B-171 3 0.00 3.68 3.73 92.28 0.30 03-B-20 4 4 0.67 4.68 5.86 88.49 0.3003-B-23 7 5 0.00 4.50 5.70 89.80 0.00 03-B-22 6 6 0.92 2.83 3.49 92.620.15 03-B-25 9 7 0.00 4.82 7.32 87.71 0.15 03-B-21 5 8 1.02 3.14 4.8790.66 0.30 03-B-18 2 9 0.43 2.97 4.56 92.05 0.00 Minimum 0.00 2.83 3.4987.71 0.00 Maximum 1.59 4.91 7.32 92.62 0.30

Insulators 14 were produced using the above-described materialcompositions. The insulators were tested for their resistance todielectric puncture. In order to test the dielectric punctureresistance, the insulators were placed in a fixture comprising a centerelectrode that passed through the axial bore of the insulator. A groundelectrode was placed around the exterior surface of the insulator at apoint where the thickness of the insulator was about 0.100 inches. Thetest fixture and insulator were immersed in a dielectric fluid toprevent arcing of the electric current around the insulator. AHipotronics dielectric tester was used to apply a 60 Hertz alternatingcurrent electrical field to the insulator. Voltage was ramped at a rateof 200 volts per second until dielectric puncture of the insulatoroccurred. The peak voltage at the time of failure was reported as thedielectric puncture voltage. The results of the testing are set forth inTable 3 below.

TABLE 3 Voltage kV 03- 03- 03- 03- 03- 03- 03- 03- 03- Composition B-17B-18 B-19 B-20 B-21 B-22 B-23 B-24 B-25 Average 40.9 36.9 37.8 35.1 37.839.3 39.2 34.5 34.9 Standard 1.7 2.0 2.5 1.7 3.0 3.0 2.1 1.8 4.1Deviation. Minimum 37.6 33.2 32.5 32.4 32.1 30.8 31.4 31.5 19.2 Maximum45.1 43.0 45.2 39.0 47.9 44.3 43.8 39.9 39.0 No. Specimens 60 58 28 2644 38 60 51 30

A main effects plot of mean dielectric puncture values is illustrated inFIG. 2 in terms of kilovolts. As can be seen, the plot of Al₂O₃ contentreveals the most significant increase in resistance to dielectricpuncture over the three levels of the Al₂O₃ variable. Accordingly, theamount of alumina is believed to be the variable with the mostsignificant effect on dielectric puncture. In general, higher aluminacontent in the ceramic tends to result in higher dielectric puncturevalues, and vice-versa. In other words, the data reveal that increasesin resistance to dielectric puncture of the ceramic are most dependentupon increases in the quantity of alumina. Other variables, such as theSiO₂:RO ratio (RO being in this instance CaO+MgO), the CaO:RO ratio, andthe amount of zirconia are not believed to have as significant of aneffect on dielectric puncture. However, since no maxima or minima wasobserved with regard to the effect of the zirconia content, and punctureperformance improved with increasing zirconia content, it is believedthat zirconia content including higher zirconia contents than thosetested, may provide a useful means for improving the dielectric punctureperformance of these ceramics. It is believed that a dielectric puncturethreshold of over 41 kilovolts may be repeatably achieved with theceramic formulations of the present invention.

The insulators were also tested for shunt resistance at 1,000 degreesFahrenheit. In order to measure shunt resistance, the insulators wereassembled into spark plugs and the ground electrodes were removed. Thespark plugs were mounted in a fixture comprising an electricallygrounded Inconel plate with threaded holes to receive the shells of thespark plugs and the fixture was placed into an electric furnace.Electrodes were placed on the terminals of each spark plug, with leadsthat passed through the door of the furnace. The furnace was heated to atemperature of 1000 degrees Fahrenheit and the resistance of each sparkplug was measured between the electrically grounded Inconel plate andthe terminal lead using a Keithley electrometer model number 6517A. Theresults of the shunt resistance testing are shown below in Table 4 andreported in megaohms.

TABLE 4 Batch Specimen #1 Specimen #2 Specimen #3 Average 03-B-17 29703840 2130 2980 03-B-18 898 1080 1740 1239 03-B-19 2060 3790 3390 308003-B-20 4920 11800 2530 6417 03-B-21 1050 8840 2700 4197 03-B-22 99003880 4880 6220 03-B-23 3550 2640 4380 3523 03-B-24 2630 482 2550 188703-B-25 2230 1020 3690 2313

A main effects plot of mean shunt resistance values is illustrated inFIG. 3. As can be seen, the plot of shunt resistance as a function ofthe SiO₂ content illustrated in the form of the SiO₂ to RO ratio (RO inthis instance being CaO+MgO) reveals the most significant effect onshunt resistance. The plot indicates a maximum effect on shuntresistance occurring at a ratio of 1.0. The amount of zirconia isbelieved to have the second largest effect on shunt resistance, with amaximum at about 0.3 weight percentage, which was the highest zirconiacontent in the test samples. However, since no clear maxima or minimawas observed, it is believed that higher zirconia contents may provideeven greater shunt resistance values. The CaO content, illustrated inthe plot in the form of the CaO to RO ratio, is believed to have thethird largest effect on shunt resistance with a maxima at a ratio ofabout 0.8. Surprisingly, in direct contrast to the dielectric breakdowntesting, the alumina content did not appear to have a significant effecton shunt resistance.

To account for the non-linearity of the SiO₂ to RO ratio plot, amultiple regression analysis was performed including an SiO₂ squaredterm for the SiO₂ to RO ratio. An initial analysis revealed that Al₂O₃was not statistically significant, so this variable was removed for afinal analysis. The results of the final analysis indicated that theR-squared value from the regression was 0.98, indicating that themultiple regression model analysis accounts for 98% of the variabilityin shunt resistance.

The influence of the composition of the matrix on the shunt resistanceof the ceramic is illustrated as a contour plot in FIG. 4. The contourplot illustrates that shunt resistance, at about 7000 megaohms, isachievable at 1000 degrees Fahrenheit with an SiO₂:RO ratio ofapproximately 1.0 and a CaO:RO ratio of about 0.8. Further, the shuntresistance has more sensitivity to changes in the molar ratio of SiO₂:ROthan the molar ratio of CaO:RO.

Similarly, FIG. 6 illustrates another contour plot of shunt resistanceas a function of the molar ratios of SiO₂ and CaO, similar to that ofFIG. 4, that is overlaid onto a portion of a phase equilibrium diagramfor CaO, SiO₂, and MgO. FIG. 5 is a schematic illustration of the CaO,SiO₂ and MgO ternary phase diagram illustrating generally the regionutilized in conjunction with the compositions of the present inventionand described in greater detail in FIG. 6. The phase diagram utilizedfor the overlay of FIG. 6 is available from the American CeramicsSociety, Columbus, Ohio. The left most boundary of the contour plotdepicts the left most boundary of the phase equilibrium diagram whereinthe amount of CaO and SiO₂ are varying as shown and the amount of MgO iszero. The right most boundary of the contour plot is delimited by a CaOto RO molar ratio of about 0.8, wherein 80% of the network modifierincludes CaO and 20% of the network modifier includes MgO. The lower andupper boundaries of the contour plot depict the SiO₂ to RO molar ratios,0.8 and 1.2 respectively. Within the linear boundaries of the contourplot, several partially elliptical contour bands of constant shuntresistance are illustrated. The bands range from about 3500 megaohms upto at least 7000 megaohms. Thus, the contour plot reveals that theceramic material composition of the present invention enables productionof a spark plug having a shunt resistance of at least 1000 megaohms at1000 degrees Fahrenheit and, so far, most preferably, up to about 7000megaohms at 1000 degrees Fahrenheit.

FIGS. 5 and 6 thus demonstrate that, based on the experimentation thusfar conducted and disclosed herein, the optimized value of shuntresistance in the ceramic tends to follow the phase equilibrium line inthe phase equilibrium diagram which extends between CaO.SiO₂ andCaO.MgO.SiO₂. Based on this experimentation, it is further believed thatthis discovery may be extrapolated entirely across the phase equilibriumdiagram along the phase equilibrium line extending between CaO.SiO₂ andMgO.SiO₂. More specifically, optimized shunt resistance is believed toexist within a bandwidth of the above-described line that can bedescribed as having a SiO₂ to RO molar ratio between about 0.8:1 and1.2:1.

Based on the experimentation above, the preferred ranges of theconstituent materials have been determined. A ceramic material including90 to 99 percent by weight of alumina, 0.01 to 1 percent by weight of azirconium-based compound, and 1 to 10% by weight of an oxide mixture ofa glass former and a network modifier, wherein the preferred molar ratioof the glass former to the network modifier ranges between about 0.8:1and 1.2:1. The zirconium-based compound is preferably zirconia (ZrO₂),but may also include various organic and inorganic compounds and/orcomplexes which contain zirconium. Zirconium containing compounds of thepresent invention may include any organic or inorganic compound orcomplex which contains zirconium and which enables zirconium to beincorporated into the oxide mixture matrix in the course of sinteringthe ceramic while also providing shunt resistance and dielectricpuncture resistance consistent with the results presented herein withrespect to the use of zirconia as the zirconium containing compound. Ashas been illustrated herein, zirconia may be utilized as the zirconiumcontaining compound of the present invention. It may be utilized byitself or in conjunction with other zirconium containing compounds asdescribed herein. It is believed that other zirconium containingcompounds of the present invention may include, for example, inorganiczirconium compounds such as zirconium orthosilicate, zirconium sulfate,zirconium nitrate, zirconium phosphide, zirconium silicide, andzirconium sulfide, as well as various organic compounds and inorganicand organic complexes which contain zirconium. The zirconium containingcompound should contain an amount of zirconium equivalent to 0.01-1.0%by weight of zirconia. Further, as discussed in the Manning patent,zirconium compounds generally contain some hafnium as an impurity. It isbelieved that hafnium and hafnia can be substituted interchangeablyherein when referring to zirconium and zirconia, respectively, and thatmixtures of zirconium-based and hafnium-based compounds may be utilizedin place of zirconium-based compounds, all within the scope of thepresent invention. Preferably, the glass former is SiO₂ and the networkmodifier is CaO, MgO, SrO, and/or BaO, but may also include alkalinemetal oxides such as Na₂O, K₂O, Li₂O and the like. More specifically,the network modifier is preferably composed primarily of CaO andsecondarily of MgO.

A more preferred range of materials includes the ceramic material havingalumina in an amount between about 94 and about 97% by weight, zirconiain an amount between about 0.1 and about 0.5% by weight, and the oxidemixture of the glass former and network modifier in an amount betweenabout 2.5 and about 5.9% by weight, wherein the molar ratio of saidglass former to the network modifier equals between about 0.9:1 and1.1:1, such that oxide mixture can be described by a molar equation asfollows:(Mg_(V)Ca_(W)Sr_(X)Ba_(Y))O.ZSiO₂  (1)wherein V+W+X+Y=1, and 0.8≦Z≦1.2, and more preferably, wherein0.9≦Z≦1.1.

An even more preferred range includes the ceramic material having thealumina in an amount between about 95 and about 96.5% by weight, thezirconia in an amount between about 0.25 and about 0.35% by weight, andthe oxide mixture in an amount between about 3.15 and about 4.75% byweight, wherein the network modifier includes CaO in an amount about 0.8by mole fraction and MgO in an amount about 0.2 by mole fraction. Thenetwork modifier includes CaO in an amount between about 1.38 and about1.95% by weight, and MgO in an amount between about 0.15 and about 0.43%by weight. The glass former comprises SiO₂ in an amount between about1.87 to about 2.28% by weight.

In one specific embodiment, the ceramic material includes the alumina inan amount of about 95.67% by weight, the zirconia in an amount of about0.31% by weight, and the oxide mixture in an amount of about 3.94% byweight. The oxide mixture includes CaO in an amount of about 1.55% byweight, MgO in an amount of about 0.27% by weight, and SiO₂ in an amountof about 2.12% by weight.

In another specific embodiment, the ceramic material includes thealumina in an amount of about 95.55% by weight, the zirconia in anamount of about 0.31%, CaO in an amount of about 2.04% by weight, SiO₂in an amount of about 2.02% by weight, and no MgO.

In yet another specific embodiment, the ceramic material includes thealumina in an amount of about 95.84% by weight, CaO in an amount ofabout 2.05% by weight, SiO₂ in an amount of about 2.03% by weight, andno zirconia.

As discussed previously, network intermediates such as Al₂O₃ may beadded to create alumino-silicate glass in order to further impede themotion of charge carriers. According to one preferred embodiment, Al₂O₃in an amount as much as 40% by weight may be added to the oxide mixture.Also, the oxide mixture may be a calcium-alumino-silicate glass with upto 10% by weight of MgO or the other alkaline earth oxides added asnetwork modifiers.

It is contemplated that the ceramic may also include various impurities,such as K₂O, TiO₂, P₂O₅, Fe₂O₃, and the like in a combined total amountof up to about between 0.01% and 0.50% by weight. Typically, however,such impurities are present in a combined total amount of about between0.07% and 0.30%.

The experimentation reveals that a maximum in shunt resistance isachievable over a temperature range of 800 to 1200 degrees Fahrenheitwhen the molar ratio of SiO₂ to network modifier is about 1 to 1.Additionally, the shunt resistance is optimized when the ratio of CaO toRO is about 0.8 and when the amount of zirconia is about 0.3% by weightof the ceramic.

Further, it is believed that the zirconia not only improves themechanical strength of the ceramic, but also improves the shuntresistance, by reducing crystallization within the matrix mixture whenthe ceramic is formed and cooled. Formation of a crystalline phase tendsto result in an increase in conductivity of the oxide matrix mixture,and an attendant decrease in shunt resistance. When zirconia is added tothe ceramic, at least a portion of the zirconia dissolves into themixture of the glass former and network modifier and reducescrystallization thereof. Therefore, by reducing the crystallizationwithin the oxide mixture matrix, the addition of zirconia tends toincrease the shunt resistance. Nonetheless, despite the addition ofzirconia, the oxide mixture matrix may contain some crystalline phasestherein.

The ceramic material composition of the present invention enables thespark plug to be operated at higher voltages and at higher operatingtemperatures due to reduced susceptibility of dielectric failure of thematerial and increased shunt resistance of the material under suchextreme conditions, thereby resulting in an attendant increase in shuntresistance of the spark plug.

The ceramic material composition of the present invention is resistantto dielectric breakdown whereby the integrity of the electricalresistivity of the material is maintained, to provide a spark plug withhigh shunt resistance.

The present invention also relates to ignition devices such as sparkplugs, igniters, and other spark generation devices. A spark plugassembly 10 is illustrated in sectional view in FIG. 1. The spark plug10 includes an outer shell 12 secured to an insulator 14. The outershell 12 includes a ground electrode 18. The insulator 14 has a centralbore 20 in which is situated a terminal 30, a conductive material 46, asealing material 48, a center electrode 32. The center electrode 32includes a tip 56 having a firing end 37 facing the ground electrode 18with a spark gap 34 therebetween. From a terminal end 11 and extendingtoward a firing end 13, the spark plug insulator 14 includes a terminalportion 52, a large shoulder 56, a small shoulder 58, and a firing endportion or core nose 54. The insulator 14 is further formed with varyingwall thicknesses between the inner surface 64 of the central bore 20 andthe outer surface 66. The inner surface 64 defines a center bore seat 68against which the center electrode 32 rests. The insulator 14 is formedfrom a material having approximately at least 88% by weight alumina(Al₂O₃) and up to 99.9% alumina, and more specifically at least 90% byweight alumina up to 99% alumina. The alumina may be present from about94% to about 96% by weight of the ceramic. To help with the sinteringprocess as well as improve the electrical and mechanical properties ofthe insulator 14, the insulator 14 is made from a material alsocontaining one or more various metal oxides and a glass. The glasstypically forms about 1-10% of the insulator.

The insulator 14 is designed to receive the electrically conductivecenter assembly 16 which is formed of the terminal 30, conductivematerial 46, sealing material 48, and center electrode 32. In operation,the center electrode 32 receives a high voltage ignition pulse from theignition system (not shown) which travels along the center wire assembly16 until it reaches the lower exposed end of the center electrode 32. Ifthe pulse has sufficient energy to bridge the spark gap 102, a spark isformed between the center electrode 50 and the ground electrode 22. Theground electrode 22 is grounded to the engine (not shown). The sparkignites a fuel/air mixture which has previously been injected into acombustion chamber within the engine. The ignition of this fuel/airmixture initiates the combustion process used to power the engine.

The insulator 14 of the present invention is an alumina-based ceramic.In general, alumina-based ceramics comprise fine crystalline Al₂O₃particles in an oxide mixture matrix. The oxide mixture is preferably agenerally amorphous glass matrix, such as various types of silicateglasses, but may also include crystalline materials as part of the oxidemixture. The ceramic material for the insulator generally includes byweight 90 to 99% alumina. To improve shunt resistance and/or dielectricstrength, the oxide mixture matrix is formed with a glass former, suchas SiO₂ and includes at least Boron Oxide (B₂O₃), Phosphorus Oxide(P₂O₅), or a combination of Boron Oxide and Phosphorus Oxide. The oxidemixture matrix may also include other network modifiers, such as CaO,MgO, BaO, and SrO, alkaline earth metal oxides, or alkali metal oxides.The alkali metal oxides may include Na₂O, K₂O, and Li₂O. Other networkintermediates such as TiO₂, ZnO, ZrO₂, and the like may be used inaddition to Al₂O3.

The amorphous glass matrix is generally formed from Silicon Dioxide,however other glass formers may be used. The amorphous glass matrix mayinclude other compounds, such as Calcium Oxide, Magnesium Oxide,Strontium Oxide, Barium Oxide, Aluminum Oxide, Zirconium Oxide,Phosphorus Oxide, Boron Oxide, Sodium Oxide, Lithium Oxide, PotassiumOxide, and similar oxides. The dielectric strength and shunt resistanceof a ceramic material such as a spark plug insulator is improved throughadding at least 0.05% Phosphorus Oxide by weight of the ceramic materialor about greater than 1% by weight of the glass. The Phosphorus Oxide isadded up to about 20% by weight of the glass and more preferably up toabout 18% by weight of the glass, or up to about 1% by weight of theceramic. In amounts over 20%, properties of the ceramic, which aredesirable for a spark plug, may decrease, such as shunt resistance anddielectric strength. It has been found that the best balance ofdesirable features may be obtained when the glass includes approximately1-18% Phosphorus Oxide and more preferably 2.5%-15% Phosphorus Oxide byweight of the glass. The highest shunt resistance values and dielectricstrength values occur when the Phosphorus Oxide forms approximately4-15% of the glass by weight and more preferably approximately 9% byweight.

Dielectric strength and shunt resistance of a ceramic, such as a sparkplug insulator may be improved also by forming the ceramic with a glassincluding approximately 2.0% or greater Boron Oxide by weight of theglass. The glass may include Boron Oxide up to 11% by weight and morepreferably up to 9% by weight. The inventors have found a good balanceof desirable characteristics in a ceramic for a spark plug insulatorthat includes approximately 2.5% to 11% Boron Oxide by weight of theglass.

Dielectric strength and shunt resistance of a ceramic, such as a sparkplug insulator may be further improved by using both Boron Oxide andPhosphorus Oxide in the glass as may be seen in Table 5 below. The glassgenerally includes approximately 0.5%-20% Phosphorus Oxide and 2.0% to11% Boron Oxide by weight of the glass. The glass forms approximately upto 12% of the ceramic material. As may be seen in Table 5, theDielectric strength and shunt resistance peak when about 10% by weightPhosphorus Oxide of the glass is added and about 4.5% by weight BoronOxide of the glass is added. The mixture contour plots, of FIGS. 7-11and especially FIG. 9 show the optimal ranges for shunt resistance anddielectric strength of a ceramic material. FIGS. 7-9 use a (MgCa) SiO₂glass matrix, while FIGS. 10 and 11 use a 3B glass matrix. The term 3Bin the Figures refers generally to a glass matrix havingMg_(0.2)Ca_(0.8)SiO₂. A 3B ceramic may also include alumina in an amountof about 95-96% and more specifically about 95.67% by weight; thezirconia in an amount of about 0.2-0.4% by weight and more specificallyabout 0.31% by weight; and the oxide mixture in an amount of about 3-5%and more specifically about 3.94% by weight. The oxide mixture generallyincludes CaO in an amount of about 1-2% and more specifically about1.55% by weight; MgO in an amount of about 0.1-0.4% by weight and morespecifically about 0.27% by weight; and SiO₂ in an amount of about1.5-2.5% and more specifically about 2.12% by weight. The “ID Number” inColumn 1 of Table 5 refers to an insulator with compositionsapproximately as calculated and shown in Table 6. Table 7 and 8 providefurther information regarding the compositions used to form the ceramic.Table 9 provides approximate Molar Percentages in Terms of End Members.

TABLE 5 Data Summary Shunt Apparent Resistance Dielectric ID (MgCa)SiO₂3CaO•P₂O₅ 2CaO•3B₂O₃ Density at 1000° F. Strength Number Mol % Mol % Mol% (g/cc) M-ohms V/mil (rms) 1 100.0 0.0 0.0 3.793  2500* 376 2 91.3 8.70.0 3.792 2994 375 3 94.1 0.0 5.9 3.803 4859 388 4 82.6 17.4 0.0 3.7822370 372 5 85.4 8.7 5.9 3.789 8245 399 6 88.1 0.0 11.9 3.793 7468 393 773.9 26.1 0.0 3.765 3555 377 8 76.7 17.4 5.9 3.773 5418 390 9 79.4 8.711.9 3.784 3971 391 10 82.2 0.0 17.8 3.792 4420 391 11 92.6 4.4 3.03.786 * 389 12 79.6 17.4 3.0 3.774 * 372 13 83.7 4.4 11.9 3.788 6547 392*Data not available due to difficulties making insulators. For #1,typical 3B data was used.

TABLE 6 Batch Information Calculated Composition Alumina ZrO₂ MgO CaOP₂O₅ B₂O₃ SiO₂ ID wgt wgt wgt wgt wgt wgt wgt Number % % % % % % % 195.79 0.31 0.27 1.51 0.00 0.00 2.12 2 95.47 0.31 0.25 1.73 0.31 0.001.93 3 95.71 0.31 0.26 1.53 0.00 0.18 2.02 4 95.15 0.30 0.22 1.95 0.620.00 1.75 5 95.39 0.31 0.23 1.75 0.31 0.18 1.83 6 95.63 0.31 0.24 1.550.00 0.35 1.91 7 94.83 0.30 0.20 2.17 0.94 0.00 1.56 8 95.07 0.30 0.211.97 0.62 0.18 1.64 9 95.31 0.30 0.22 1.77 0.31 0.35 1.73 10 95.54 0.310.23 1.58 0.00 0.53 1.81 11 95.59 0.31 0.25 1.63 0.16 0.09 1.98 12 95.110.30 0.22 1.96 0.62 0.09 1.70 13 95.47 0.31 0.23 1.66 0.16 0.35 1.82

TABLE 7 Batch Information Batches Used Calcium ID Talc Calcium Phos-Calcium EPK Num- wgt Carbonate phate Borate wgt Alumina Zirconia ber %wgt % wgt % wgt % % wgt % wgt % 1 0.88 2.72 0.00 0.00 3.35 92.75 0.30 20.80 2.48 0.66 0.00 3.06 92.70 0.30 3 0.84 2.58 0.00 0.37 3.19 92.720.30 4 0.73 2.24 1.31 0.00 2.77 92.65 0.30 5 0.76 2.35 0.66 0.37 2.9092.67 0.30 6 0.79 2.45 0.00 0.73 3.02 92.70 0.30 7 0.65 2.01 1.97 0.002.48 92.59 0.30 8 0.68 2.11 1.31 0.37 2.61 92.62 0.30 9 0.72 2.21 0.660.73 2.73 92.65 0.30 10 0.75 2.32 0.00 1.10 2.86 92.67 0.30 11 0.82 2.530.33 0.18 3.12 92.71 0.30 12 0.71 2.18 1.31 0.18 2.69 92.63 0.30 13 0.762.33 0.33 0.73 2.88 92.67 0.30

TABLE 8 Batch Information Unity Formula Of Glass ID MgO CaO P₂O₅ B₂O₃SiO₂ Number Moles Moles Moles Moles Moles 1 0.20 0.80 0.00 0.00 1.06 20.17 0.83 0.05 0.00 0.91 3 0.19 0.81 0.00 0.08 1.01 4 0.14 0.86 0.100.00 0.76 5 0.16 0.84 0.05 0.08 0.85 6 0.18 0.82 0.00 0.15 0.95 7 0.110.89 0.15 0.00 0.60 8 0.13 0.87 0.10 0.08 0.70 9 0.15 0.85 0.05 0.150.80 10 0.17 0.83 0.00 0.23 0.90 11 0.18 0.82 0.03 0.04 0.96 12 0.140.86 0.10 0.04 0.73 13 0.17 0.83 0.03 0.15 0.88

TABLE 9 Molar Percentage In Terms Of End Members(MgO_(0.2)CaO_(0.8))SiO₂ 3CaO•P₂O₅ 2CaO•3B₂O₃ ID Number Mol % Mol % Mol% 1 100.0 0.0 0.0 2 91.3 8.7 0.0 3 94.1 0.0 5.9 4 82.6 17.4 0.0 5 85.48.7 5.9 6 88.1 0.0 11.9 7 73.9 26.1 0.0 8 76.7 17.4 5.9 9 79.4 8.7 11.910 82.2 0.0 17.8 11 92.6 4.4 3.0 12 79.6 17.4 3.0 13 83.7 4.4 11.9

As shown in the tables, the optimum range is about 0.08 to 0.15 moles ofB₂O₃ and less than 0.10 moles of P₂O₅. In terms of overall formulation,the range is approximately 0.18 to 0.35% by weight Boron Oxide and up to0.62% by weight Phosphorus Oxide.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is, therefore, to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described. The inventionis defined by the claims.

1. A spark plug having an insulator, the insulator comprising: aluminain an amount of approximately 90 to 99% by weight of the insulator; anda glass including about 1% to 18% Phosphorus Oxide by weight of theglass.
 2. The spark plug of claim 1 wherein the glass includes by weightapproximately 2.5% to 15% Phosphorus Oxide.
 3. A spark plug having aninsulator, the insulator comprising: alumina in an amount ofapproximately 90 to 99% by weight of the insulator; and a glassincluding about 0.05% or greater Phosphorus Oxide by weight of the glassand greater than 0.5% Boron Oxide by weight of the glass.
 4. The sparkplug of claim 3 wherein the glass includes by weight greater than 1%Boron Oxide.
 5. The spark plug insulator of claim 3 wherein the glassincludes by weight less than 15% Boron Oxide.
 6. The spark plug of claim5 wherein the glass includes by weight about 2.5% to 11% Boron Oxide. 7.The spark plug of claim 6 wherein the glass includes by weightapproximately 4 to 6% Boron Oxide.
 8. The spark plug of claim 7 whereinthe glass includes by weight approximately 5.5% Boron Oxide.
 9. A sparkplug having an insulator, the insulator comprising: alumina in an amountof approximately 90 to 99% by weight of the insulator; and a glassincluding Phosphorus Oxide and approximately 2.5% to 11% Boron Oxide byweight of the glass.
 10. The spark plug of claim 9 wherein the glassincludes by weight Phosphorus Oxide in the amount of approximately 1.0%to 18%.
 11. The spark plug of claim 10 wherein the glass includes byweight Phosphorus Oxide in the amount of approximately 9% and BoronOxide in the amount of approximately 4-6%.